1. Field of the Invention
The present invention relates to a transport speed control system for a flat glass annealing lehr provided at an intermediate position along a glass ribbon production line and including a plurality of glass ribbon transport rollers in juxtaposition.
2. Description of the Prior Art
FIG. 2 of the accompanying drawings illustrates an example of transport speed control system for the glass ribbon annealing installation of the type noted above. As seen, the speed control system includes a motor 52 for driving all glass ribbon transport rollers 50 simultaneously through a speed reducer 51, a drive speed sensor 53 for detecting peripheral speeds of rollers 50 on the basis of rotational rates of the speed reducer 51, a drive speed setter 54 for setting a peripheral speed of all the rollers 50, and a controller 55 responsive to a detection signal from the sensor 53 and a speed setting signal from the speed setter 54 for outputting a control signal to the motor 52 to drive the rollers 50 at the set peripheral speed.
Such a known transport speed control system is designed to drive all of the rollers 50 at an equal peripheral speed in order to transport a glass ribbon in a stree-free state. However, the rollers in an upstream part of the furnace with respect to the direction of transport are exposed to a higher glass ribbon temperature than downstream rollers, and therefore a greater thermal expansion and a greater eccentricity occur with the upstream rollers. Besides, salt cakes adhere to peripheral surfaces of the rollers 50 in an increasing amount as a glass ribbon is transported. These phenomena positively cause the upstream rollers to rotate at a higher peripheral speed than the downstream rollers, with the result that the transported glass ribbon is constantly subjected to a compressive force. While the transported glass ribbon has a high temperature in the range of 500.degree.-600.degree. C. and remains highly viscous and soft, for example, the glass ribbon tends to warp into a permanent deformation owing to an internal stress. On the other hand, the transported glass ribbon at a lower temperature could develop cracks.
Furthermore, since there is a difference between a peripheral speed of the rollers and the glass ribbon transport speed, any projections on roller surfaces may abrade the glass ribbon.
The above problems, particularly the problem of deformation and cracking, are the more conspicuous the thinner the transported glass ribbon is.